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Spatio-Temporal Rivalry

Recently we discovered a phenomenon, we call Spatio-Temporal Rivalry (STR), wherein exposure to a physically coherent moving object elicits two conflicting illusory percepts.

The stimulus used in reported experiments consisted of a rotating windmill. The arms of the windmill either had a single green stripe at the mid-point of otherwise red arms, or consisted of multiple red and green stripes.

With the first type of stimulus, STR is characterised by mutually exclusive intermittent appearences of an illusory static green ring at the mid-point of the windmill arms, and by intermittent dissapearences of all trace of green, leaving an impression of an illusory red rotating windmill.

Click here to see an interactive demonstration
CAVEAT: The online demonstration is suboptimal. Optimal effects depend on high rates of local repetition (~10Hz) and on equating red and green luminance. We cannot ensure either status over the web, but the effect should be sufficiently robust for you to get some idea of the perceptual effects observed. Also, due to the poor sampling rate available over the web, we have used a sinusoidal luminance modulated windmill, as opposed to the square wave pattern used in reported experiments.

With the second striped stimulus STR is characterised by mutually exclusive intermittent appearences of an illusory static striped surface (a bit like a striped doughnut) and by an intermittent impression of an illusory yellowey-red rotating windmill.

Click here to see an interactive demonstration
CAVEAT: This online demonstration is even more suboptimal. Even in ideal situations these effects occur less frequently. They also depend on high rates of local repetition (~10Hz) and on equating red and green luminance. However, the effects should be sufficiently robust for you to get some idea of the perceptual effects observed in laboratory conditions.

TIP: Look at the changed configuration for about 10 seconds then change back to the original configuration for best effects!

We believe these illusions occur because human vision contains different mechanisms optimised for processing static and moving inputs. Mechanisms optimised for encoding static input are sensitive to thin coloured stimulus regions, but integrate information relatively slowly. So, like a camera with a slow shutter speed, when inputs move rapidly they can prompt these mechansisms to signal the presence of a blurred form elongated along the trajectory of motion. We propose that in our rapidly moving repetitive stimulus, this creates the impression of illusory static stripes due to physically offset elements becoming perceptually fused due to blur signals.

We believe that the illusory impression of a single coloured rotating surface is generated by mechanisms optimised for encoding movement. These seem to be involved in generating an impression of a rotating surface wherein apparent colour is determined by a spatial averaging process (resulting in red for the first stimulus and yellowey-red for the second).